Electric actuator for a marine steering system

ABSTRACT

An electric actuator comprises a housing and an output shaft reciprocatingly received by the housing. There is a screw assembly disposed within the housing and coupled to the output shaft. The screw assembly includes a plurality of annular rollers and a central screw received by the annular rollers. The annular rollers are rotatable about the central screw. There is a motor which includes a stator and a rotor. The rotor has an inner bore which engages the annular rollers. Rotation of the rotor causes the central screw to translate axially relative to the rotor and the output shaft to reciprocate relative to the housing.

FIELD OF THE INVENTION

The present invention relates to an electric actuator and, inparticular, to an electric actuator for a marine steering system.

BACKGROUND OF THE INVENTION

U.S. Patent Application Publication No. 2005/0170713, which waspublished on Aug. 4, 2005 in the name of Okuyama, discloses an electricactuator for a marine steering system. The electric actuator includes ahousing having a shaft in the form of a ball screw extendingtherethrough. There is a DD (direct drive)-type motor disposed withinthe housing and mounted about the ball screw. Rotation of the DD-typemotor about the ball screw results in the DD-type motor translatingaxially relative to the ball screw. This in turn results in the housingreciprocating relative to the ball screw. Securing ends of the ballscrew to support brackets and coupling the housing to a tiller of apropulsion unit results in steering motion being imparted to thepropulsion unit as the housing reciprocates relative to the ball screw.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an electric actuatorfor a marine steering system.

There is accordingly provided an electric actuator for a marine steeringsystem which comprises a housing and an output shaft reciprocatinglyreceived by the housing. There is a screw assembly disposed within thehousing and coupled to the output shaft. The screw assembly includes aplurality of annular rollers and a central screw received by the annularrollers. The annular rollers are rotatable about the central screw.There is a motor which includes a stator and a rotor. The rotor has aninner bore which engages the annular rollers. Rotation of the rotorcauses the central screw to translate axially relative to the rotor andthe output shaft to reciprocate relative to the housing.

There is also provided an electric actuator for a marine steering systemwhich comprises a housing and an output shaft reciprocatingly receivedby the housing. There is a screw assembly disposed within the housingand coupled to the output shaft. The screw assembly includes a nut and acentral screw received by the nut. The nut is rotatable about thecentral screw. There is a motor which includes a stator and a rotor. Therotor has an inner bore which engages the nut. Rotation of the rotorcauses the central screw to translate axially relative to the rotor andthe output shaft to reciprocate relative to the housing.

There may be an adjustable clutch coupled to the rotor which inhibitsrotation of the rotor. The output shaft may be rotatable relative to therotor to release the clutch.

The clutch may include an electromagnet fixed to the housing and aclutch plate translatable axially relative to the housing. The clutchplate may be preloaded to be spaced-apart from the electromagnet andabutting the rotor. Friction between the clutch plate and the rotor mayinhibit relative rotation of the rotor. The clutch may be disengaged bypowering the electromagnet to attract the clutch plate towards theelectromagnet, creating an air gap between the clutch plate and therotor, and thereby allowing the rotor to rotate freely.

The clutch may alternatively include a permanent magnet fixed to thehousing and a clutch plate translatable axially relative to the housing.The clutch plate may be attracted by the permanent magnet to abut thehousing. Friction between the clutch plate and the housing may inhibitrelative rotation of the rotor. The clutch may include anelectromagnetic coil. The clutch may be disengaged by powering theelectromagnetic coil to cancel the force of the permanent magnet,thereby allowing the clutch plate to rotate relative to the housing andthe rotor to rotate freely. There may be a spring that biases the clutchplate away from the housing when the electromagnetic coil is powered.

The electric actuator may include end glands which engage the outputshaft to minimize any bending load from being transferred to the screwassembly. A cogging torque of the electric actuator may be tuned to slipat a predetermined back-driving force to function as a clutch.

There is further provided a steering system for a marine vessel having apropulsion unit. The steering system comprises an electric actuator forimparting steering movement to the propulsion unit. The electricactuator includes a housing and an output shaft reciprocatingly receivedby the housing. There is a screw assembly disposed within the housingand coupled to the output shaft. The screw assembly includes a pluralityof annular rollers and a central screw received by the annular rollers.The annular rollers are rotatable about the central screw. There is amotor which includes a stator and a rotor. The rotor has an inner borewhich engages the annular rollers. Rotation of the rotor causes thecentral screw to translate axially relative to the rotor and the outputshaft to reciprocate relative to the housing.

The propulsion unit may include a tilt tube and a support rod receivedby the tilt tube. The electric actuator may include support arms whichconnect respective ends of the output shaft to the support rod of thepropulsion unit. The support arms may inhibit axial movement of theoutput shaft relative to the marine vessel while the housing of theelectric actuator reciprocates along the output shaft and linearlyrelative to the marine vessel.

The propulsion unit may include a tiller and the electric actuator mayinclude a pivot plate pivotably connected to the tiller. The pivot platemay rotationally constrain the housing of the electric actuator toprovide reaction torque for rotation of the rotor.

There may be an impact absorber to adjust a stiffness of the electricactuator. The electric actuator may have a longitudinal axis and theimpact absorber may be positioned to permit compliance in a directionparallel to the longitudinal axis.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more readily understood from the followingdescription of the embodiments thereof given, by way of example only,with reference to the accompanying drawings, in which:

FIG. 1 is a top plan view of a first embodiment of an electric actuatorfor a marine steering system with the marine steering system shown infragment;

FIG. 2 is a rear elevation view of the first embodiment of the electricactuator;

FIG. 3 is a sectional view of the first embodiment of the electricactuator;

FIG. 4 is a fragmentary, sectional view of a second embodiment of anelectric actuator for a marine steering system;

FIG. 5 is a sectional view of a third embodiment of an electric actuatorfor a marine steering system; and

FIG. 6 is a sectional view of a fourth embodiment of an electricactuator for a marine steering system;

FIG. 7 is a sectional view of a fifth embodiment of an electric actuatorfor a marine steering system;

FIG. 8 is a perspective view of a brake assembly of the fifth embodimentof the electric actuator; and

FIG. 9 is a sectional view of the brake assembly of FIG. 8.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the drawings and first to FIGS. 1 and 2, there is shown afirst embodiment of an electric actuator 20 for a marine steeringsystem. The electric actuator 20 generally comprises a housing 22 withoutput shafts 24 a and 24 b reciprocatingly received therein andspaced-apart housing arms 26 and 28 which extend radially outward of thehousing 22. The output shafts 24 a and 24 b are at least partiallythreaded in this example. However, respective inner portions 27 and 29of the output shafts 24 a and 24 b are smooth so they can be sealed. Apivot plate 30 can be coupled to each of the housing arms 26 and 28 byrespective pivot pins 32 and 34. The pivot plate 30 extends between thehousing arms 26 and 28 and can pivot about the pivot pins 32 and 34. Asteering member or tiller 36 of a propulsion unit 37 can be pivotablyconnected to the pivot plate 30 by a tiller pin or bolt 38. There may bean impact absorber between the pivot plate 30 and the tiller bolt 38.For example, a slot 39 in the pivot plate 30 which receives the tillerbolt 38 may be lined with a rubber bushing or equivalent.

Support arms 40 and 42 connect respective ends of the output shafts 24 aand 24 b to a support rod 44 of a tilt tube 46 of the propulsion unit.The support arms 40 and 42 inhibit axial movement of the output shafts24 a and 24 b relative to a marine vessel (not shown) while the housing22 reciprocates along the output shafts and linearly relative to themarine vessel. This relative linear movement of the housing 22 causesthe tiller 36 of the propulsion unit to pivot and thereby causes thepropulsion unit to be steered in a conventional manner. There may alsobe impact absorbers 45 and 47 mounted on the support rod 44 of the tilttube 46. The impact absorbers can be mounted between the pivot plate 30and housing arms 26 and 28 or between the output shaft 24 a and thesupport arm 40 and between the output shaft 24 b and the support arm 42.In general, the impact absorbers may be mounted in positions that permitcompliance in a direction parallel to a longitudinal axis of theelectric actuator. The impact absorbers 45 and 47 together with anadjuster nut 49 may be used to adjust a stiffness of the electricactuator 20. In this example, the impact absorbers 45 and 47 are springwashers but may be rubber grommets or any other suitable compressiblematerial. The support arms 40 and 42 provide rotational constraint tothe output shafts 24 a and 24 b. Alternatively, this constraint could beachieved by other means such as a pin. The pivot plate 30 attached tothe tiller 36 provides rotational constraint on the housing 22. Thisprovides reaction torque for the rotation of the motor.

The electric actuator 20 is shown in greater detail in FIG. 3. Theoutput shafts 24 a and 24 b are reciprocatingly received by the housing22 and are coupled by a roller screw assembly 56. End glands 48 and 50are each provided with respective seals 52 and 54 which seal againstcorresponding smooth surfaces 51 and 53 of the output shafts 24 a and 24b and seal the housing 22. There are O-rings 53 and 55 which sealbetween the glands and the housing. The end glands 48 and 50 act asbushings with a tight clearance fit on the output shafts 24 a and 24 band function to minimize any bending load or deflection from beingtransferred to the roller screw assembly 56. The roller screw assembly56 has a central screw 58 which is received by a plurality of annularrollers 60. The rollers 60 are able to rotate about the central screw 58in a planetary fashion but do not translate axially relative to thecentral screw 58. Alignment of the rollers 60 and the central screw 58is maintained through the use of interlocking gear teeth 61 and 63 onboth the rollers 60 and the central screw 58. There are annular endplates 62 and 64 which hold the roller screw assembly 56 together. Theend plates 62 and 64 are free to rotate relative to the central screw 58and the end plates 62 and 64 are each provided with journal bearingbores 65 and 67 that allow the rollers 60 to rotate independently of theend plates 62 and 64. The end plates 62 and 64 are axially constrainedby the annular rollers 60 and the output shafts 24 a and 24 b.Alternatively, the output shafts 24 a and 24 b and the central screw 58can be made as one piece.

The rollers 60 are rotated by a motor assembly 66 which, in thisexample, is a hollow DC motor including a stator 68 having electricalwindings, a rotor 70 having magnets, and a Hall Effect sensor board 72.The electrical windings are rigidly attached or timed directly to thehousing 22. The magnets are bonded directly to the rotor 70 which isconstrained axially within the housing 22 but is able to rotate throughthe provision of bearings 74 and 76 disposed at opposite ends of therotor 70. The Hall Effect sensor board 72 is bonded or timed directly tothe stator 68 and sequences electrical power to the electrical windingsof the stator depending on a relative position of the rotor 70. Therotor 70 has a threaded inner bore 78 which threadedly engages therollers 60. Rotation of the rotor 70 relative to the roller screwassembly 56 causes the output shafts 24 a and 24 b to reciprocaterelative to the housing 22. When axial movement of the output shafts 24a and 24 b is inhibited, for example by support arms in a marinesteering system, the result is relative reciprocation of the housing 22along the output shafts 24 a and 24 b which imparts steering motion to apropulsion unit. Cogging torque of the electric actuator may be tuned toslip at a predetermined back-driving force to function as a brake orclutch.

Stroke length of the output shafts 24 a and 24 b is maximized by havingthe central screw 58 translate axially relative to the rotor 70 ascompared to conventional electric actuators in which a rotor or nuttranslates axially relative to a screw. The maximized stroke lengthallows the electric actuator to have a smaller envelope and fit withinthe geometric constraints of a marine steering system. With thisarrangement, the total length of the electric actuator relative to itsstroke length ratio can be less than three for a balanced, sealedcylinder with smooth shafts.

A second embodiment of an electric actuator 80 for a marine steeringsystem is shown in FIG. 4. The second embodiment of the electricactuator 80 is generally similar to the first embodiment of the electricactuator 20 with the exception that the second embodiment of theelectric actuator 80 is further provided with a brake or clutch 82including an electromagnet 84 and a clutch plate 86 made of ferrousmaterial. The electromagnet 84 is fixed to a housing 88 of the electricactuator 80. The clutch plate 86 is able to translate axially relativeto the housing 88. The clutch plate 86 has a tapered surface 90 whichabuts a corresponding tapered surface 92 of a rotor 94 of the electricactuator 80. The clutch plate 86 is preloaded to be spaced apart fromthe electromagnet 84 and abutting the tapered surface 92 of the rotor94. Preloading is accomplished using an O-ring 95 in this example butany suitable nonferrous deformable material such as a stainless steelspring may be used. Friction between the tapered surface 90 of theclutch plate 86 and the tapered surface 92 of the rotor 94 inhibitsrelative rotation of the rotor 94 when the clutch plate 86 is preloaded.The clutch 82 is disengaged by powering the electromagnet 84 which thenattracts the clutch plate 86 to create an air gap between the clutchplate 86 and the rotor 94 and thereby allow the rotor to rotate freely.

A slip plate 96 is used to inhibit relative rotation of the clutch plate86. The slip plate 96 is coupled to the clutch plate 86 by dowel pins,for example dowel pin 98, which are in the slip plate 96 about itscircumference. Apertures, for example aperture 100, about acircumference of the clutch plate 86 slidingly receive the dowel pinsand allow the clutch plate 86 to move axially relative to the slip plate96 as the clutch 82 is disengaged or released. Relative rotation of theslip plate 96 is inhibited by a friction shoe 102 that frictionallyengages the slip plate 96. An externally adjustable preload screw 104and spring 106 allow for friction at the slip plate 96 to be adjusted.When in use, at back-driving forces below a threshold, torque isresisted by the frictional engagement of the friction shoe 102 and theslip plate 96. At back-driving forces that exceed the threshold, theslip plate 96, the clutch plate 86, and the rotor 94 all rotate relativeto the friction shoe 102.

There is an air gap 108 between the electromagnet 84 and the clutchplate 86 when the clutch 82 is disengaged as shown in FIG. 4. The airgap 108 determines the required strength and corresponding size of theelectromagnet. Due to manufacturing tolerances, it can be difficult toattain repeatable air gaps and, as a result, oversized electromagnetsmay be required. However, the air gap 108 may be adjusted using aretaining nut 109 shown in FIG. 4. This allows the air gap 108 to be setupon assembly by positioning the rotor 94 relative to the clutch 82 andthereby reduce tolerance stack-up concerns. There is also a manualbypass mechanism which can release the clutch 82 to allow an operator tomanually reposition the electric actuator 80. A manual bypass screw 110moves the clutch plate 86 away from the rotor 94 when screwed into theelectric actuator 80. This allows the rotor 94 to rotate freely and theelectric actuator 80 to be back-driven and manually repositioned. In amarine steering system, the manual bypass mechanism may further includeremovable pins on the support brackets which would allow an output shaft112 of the electric actuator 80 to be rotated relative to the rotor 94,resulting in relative axial translation of the output shaft 112.Alternatively, the manual bypass mechanism may include nuts on ends ofthe output shaft 112 which would allow the output shaft to be rotatedrelative to the rotor 94. However, rotational constraint of the outputshaft 112 may not be required.

A third embodiment of an electric actuator 120 for a marine steeringsystem is shown in FIG. 5. The third embodiment of the electric actuator120 is generally similar to the first embodiment of the electricactuator 20 with the exception that the third embodiment of the electricactuator 120 is provided with a ball screw assembly 122 in place of theroller screw assembly. The ball screw assembly 122 includes a ball screw124 and a nut 126.

A fourth embodiment of an electric actuator 130 for a marine steeringsystem is shown in FIG. 6. The fourth embodiment of the electricactuator 130 is generally similar to the first embodiment of theelectric actuator 20 with the exception that the fourth embodiment ofthe electric actuator 130 is provided with a standard screw assembly 132in place of the roller screw assembly. The standard screw assembly 132includes a standard screw 134 and a nut 136.

A fifth embodiment of an electric actuator 140 for a marine steeringsystem is shown in FIG. 7. The fifth embodiment of the electric actuator140 is generally similar to the first embodiment of the electricactuator 20 with the exception that the fifth embodiment of the electricactuator 140 is provided with a brake or clutch 142 which is shown ingreater detail in FIGS. 8 and 9. The clutch 142 is an electromagneticclutch and includes a permanent magnet 144. In the unpowered state,magnetic fields from the permanent magnet 144 attract a clutch plate 146towards an inner face 148 of a housing 150 of the clutch 142. Frictionbetween the clutch plate 146 and the housing 150 inhibits relativerotation of the clutch plate when the clutch 142 is in the unpoweredstate. Powering an electromagnetic coil 152 in the clutch 142 creates amagnetic field that cancels the force of the permanent magnet 144,allowing the clutch plate 146 to rotate relative to the housing 150. Theclutch plate 146 is connected to a hub 154 via springs, for example,spring 156, which provide axial movement relative to one another. Thespring 156 biases the clutch plate 146 away from the inner face 148 ofthe housing 150 when the clutch 142 is powered. The hub 154 isrotationally constrained to the rotor. When the clutch 142 is powered,the rotor is able to rotate freely. When the clutch 142 is unpowered, abraking torque is applied to prevent the rotor from rotating.

It will be understood by a person skilled in the art that many of thedetails provided above are by way of example only, and are not intendedto limit the scope of the invention which is to be determined withreference to the following claims.

What is claimed is:
 1. An electric actuator for a marine steeringsystem, the electric actuator comprising: a housing; an output shaftreciprocatingly received by the housing; a screw assembly disposedwithin the housing and coupled to the output shaft, the screw assemblyincluding a nut and a central screw received by the nut, the nut beingrotatable about the central screw but the nut does not translate axiallyrelative to the central screw; and a motor including a stator and arotor, the rotor having an inner bore which engages the nut, whereinrotation of the rotor causes the central screw to translate axiallyrelative to the rotor and the output shaft to reciprocate relative tothe housing.
 2. The electric actuator as claimed in claim 1 furtherincluding an adjustable clutch coupled to the rotor which inhibitsrotation of the rotor.
 3. The electric actuator as claimed in claim 2wherein the clutch includes an electromagnet fixed to the housing and aclutch plate translatable axially relative to the housing, the clutchplate being preloaded to be spaced-apart from the electromagnet andabutting the rotor, wherein friction between the clutch plate and therotor inhibits relative rotation of the rotor.
 4. The electric actuatoras claimed in claim 3 wherein the clutch is disengaged by powering theelectromagnet to attract the clutch plate towards the electromagnet,creating an air gap between the clutch plate and the rotor, and therebyallowing the rotor to rotate freely.
 5. The electric actuator as claimedin claim 3 wherein the clutch includes a permanent magnet fixed to thehousing and a clutch plate translatable axially relative to the housing,the clutch plate being attracted by the permanent magnet to abut thehousing, wherein friction between the clutch plate and the housinginhibits relative rotation of the rotor.
 6. The electric actuator asclaimed in claim 5 wherein the clutch includes an electromagnetic coiland wherein the clutch is disengaged by powering the electromagneticcoil to cancel the force of the permanent magnet, thereby allowing theclutch plate to rotate relative to the housing and the rotor to rotatefreely.
 7. The electric actuator as claimed in claim 6 further includinga spring that biases the clutch plate away from the housing when theelectromagnetic coil is powered.
 8. The electric actuator as claimed inclaim 3 wherein the output shaft is rotatable relative to the rotor torelease the clutch.
 9. The electric actuator as claimed in claim 2further including end glands which engage the output shaft to minimizeany bending load from being transferred to the screw assembly.
 10. Theelectric actuator as claimed in claim 2 wherein a cogging torque of theelectric actuator is tuned to slip at a predetermined back-driving forceto function as a clutch.